1. Field of the Invention
The present invention relates to a method of adjusting spherical aberration and focus offset in the field of recording information onto or reproducing information from an information recording medium, such as an optical disk, and an information recording/reproduction apparatus using the same.
2. Description of the Related Art
There has been an increasing demand for optical disks having a high recording density to be used in an information recording/reproduction apparatus, in response to the increase in recent years of the quantity of information to be recorded and reproduced by such an apparatus. Attempts have been made to increase the recording density of optical disks by raising the linear recording density on the information recording layer and reducing the pitch of arrangement of the tracks of the optical disk. In order to raise the recording density of an optical disk, it is necessary to reduce the diameter of the light beam converged to the information recording layer of the optical disk.
Techniques that can be used to reduce the diameter of a light beam to be irradiated onto an optical disk include increasing the numerical aperture (NA) of the objective lens of the focusing optical system in the optical pickup for recording information onto and reproducing information from the optical disk, and decreasing the wavelength of the light beam. With regard to reducing the wavelength of the light beam, it is believed to be possible to reduce the wavelength by replacing a red semiconductor laser, which is being used as a light source, with a blue/purple semiconductor laser, and practical applications of the blue/purple semiconductor lasers have been and are being implemented.
With regard to realizing an objective lens having a high numerical aperture, on the other hand, techniques of combining two semispherical lenses to form (a lens group for) the objective lens and those of using a newly developed single lens have been proposed.
Generally, the information recording layer of an optical disk is coated by a cover layer in order to protect the information recording layer against dust and scratches. Therefore, the light beam coming out from the objective lens becomes converged and focused onto the information recording layer lying under the cover layer. As the light beam passes through the cover layer, spherical aberration occurs due to the variance of the thickness of the cover layer.
The spherical aberration under consideration is expressed by a formula of spherical aberration∝t×NA4 (t: the thickness of the cover layer, NA: the numerical aperture of the objective lens). Namely, the spherical aberration is proportional to the thickness t of the cover layer and the fourth power of the NA. Normally, the objective lens is designed to offset the spherical aberration, so that the light beam that passes through the objective lens and the cover layer is converged onto the information recording layer, because the spherical aberration is sufficiently small.
However, if the thickness of the cover layer shifts from the predetermined value, spherical aberration takes place to the light beam converged onto the information recording layer, to consequently give rise to fluctuations of the thermal distribution in a recording operation, and to reduce the resolution in a replay operation, so as to disable the ability to correctly write and read information.
The spherical aberration that arises due to the error Δt of the thickness of the cover layer is proportional to the error Δt of the thickness of the cover layer. In other words, the spherical aberration increases as the error Δt of the thickness of the cover layer increases. Additionally, since the spherical aberration is proportional to the fourth power of the NA, the spherical aberration becomes more remarkable when the NA is increased, provided that the error Δt of the thickness of the cover layer remains as it was. Then, the risk that information is neither written nor read correctly will be increased.
Among conventional optical disks, the numerical aperture NA of the objective lens of the optical pickup for DVDs (digital versatile disks) is about 0.6. Therefore, the spherical aberration that arises due to the error Δt of the thickness of the cover layer is relatively small, and it is possible to satisfactorily focus the light beam onto the information recording layer.
Meanwhile, multilayer optical disks have been developed for the purpose of increasing the density of recording information in the direction of the height of the optical disk. By definition, such a multilayer optical disk has a multiple information recording layer. For instance, DVDs having two information recording layers have already been marketed. In such a multilayer optical disk, when a number of information recording layers are laid one on the other, the distance from the surface of the optical disk (the surface of the cover layer) to the information recording layer differs from one information recording layer to another information recording layer. Then, as a result, the spherical aberration that arises when the light beam passes through the cover layer of the optical disk differs from the one information recording layer to the other information recording layer.
In the above-described arrangement, the difference of spherical aberration that appears between adjacently located information recording layers is proportional to the interlayer distance t of the adjacent information recording layers, as pointed out above. Thus, spherical aberration arises, which corresponds to the interlayer distance t. However, as far as the current standards for DVD optical systems and the interlayer distance of DVDs are observed, it is possible for DVDs to maintain the required recording/reproduction characteristics without particularly correcting the spherical aberration.
On the other hand, DVD suppliers are promoting technological developments of increasing the recording density of DVDs. The wavelength of the light source will be about 405 nm, and the NA of the objective lens will be 0.85 for DVDs that are required to accommodate such a high recording density. As pointed out above, the spherical aberration becomes more remarkable when the NA is increased, provided that the error Δt of the thickness of the cover layer remains as it is. Since spherical aberration is proportional to the fourth power of the NA, spherical aberration will be about four times greater when NA=0.85 than when NA=0.6, if the error Δt of the thickness of the cover layer is the same. In other words, the spherical aberration that arises due to the error Δt of the thickness of the cover layer becomes highly remarkable, as the NA is raised to as high as NA=0.85.
A similar problem arises for optical disks having a multilayer recording layer. The spherical aberration gives rise to a variance to a large extent as the NA of the objective lens of the optical head increases, provided that the interlayer distance t of adjacent information recording layers remains as it is. For example, the variance of spherical aberration will be about four times greater when NA=0.85 than when NA=0.6, if the interlayer distance t is the same. In other words, the variance of spherical aberration that arises due to the interlayer distance becomes highly remarkable as the NA is raised to as high as NA=0.85.
Thus, the influence of the error of spherical aberration is no longer negligible for an objective lens showing a high NA, because such an error gives rise to the problem of degraded accuracy for both recording and reading information. Then, it becomes necessary to correct the spherical aberration when using an objective lens showing a high NA, in order to realize a high recording density. For this reason, efforts are being paid for research and development of spherical aberration correction mechanisms and spherical aberration correction units.
Additionally, for reproducing information from an optical disk by means of an information recording/reproduction apparatus, it is necessary for the optical head of the apparatus to form a micro beam spot that shows a constant profile when moving along the information track on the optical disk. Therefore, the optical head is adapted to focus servo operations and tracking servo operations. A focus servo operation is a follow-up operation of driving the objective lens in a direction perpendicular to the optical disk, mainly under control, in order to minimize the beam spot. A tracking servo operation is an operation of driving the minimized beam spot to follow the information track under control.
The depth of focus of light irradiated and focused on the information recording surface of an optical disk is proportional to the wavelength λ of light and inversely proportional to the square of the numerical aperture NA of the objective lens (the depth of focus∝λ/NA2).
Accordingly, in an optical system where the wavelength of the light beam is reduced, and the objective lens is made to show a large NA in order to improve the recording density, the depth of focus is remarkably reduced as compared with a comparable optical system for currently available DVDs. Therefore, the focus servo system is required to show an excellent follow-up performance.
Thus, it is proposed for optical disk systems of the next generation to introduce a mechanism for correcting the spherical aberration in order to absorb the influence of the error of the thickness of the cover layer that arises when a multilayer recording layer is employed. In other words, it is necessary for a recording/reproduction apparatus to control and to adjust the spherical aberration correction mechanism in order to absorb the influence of the error of the thickness of the cover layer and the difference of the thickness of the cover layer that arises when a multilayer recording layer is employed.
Additionally, the change of the profile of the light spot due to spherical aberration and that of the profile of the light spot due to a focus servo offset show a complementary relationship. Therefore, it is necessary not only to improve the accuracy of focus control, but also, to optimally correct the spherical aberration and to adjust the focus offset under control simultaneously.
Known techniques for optimally adjusting the offset of the focus servo, and also, the spherical aberration correction mechanism in an optical disk system using a high NA object lens include, for instance, the one disclosed in Japanese Patent Application Laid-Open No. 2004-145987.
The above-cited patent document describes a technique of adjusting both the spherical aberration and the focus offset that arise in an optical system by detecting the optimum point for them, using a single evaluation index. More specifically, a spherical aberration correction value is made to vary under the condition of having a predetermined focus offset and the evaluation index value is detected for the obtained various spherical aberration correction values. Then, the spherical aberration correction value that makes the evaluation index best is detected among them, to optimize the spherical aberration correction value.
Thereafter, a focus offset is made to appear under the condition where the optimum spherical correction value is detected, and the evaluation index is determined to be good or bad for the focus offset value, in order to optimize the latter value. Alternatively, the spherical aberration correction value detection process and the focus offset detection process are executed in the inverse order, and the optimum spherical aberration value and the optimum focus offset value are applied as correction values of the drive apparatus.
However, with the technique disclosed in Japanese Patent Application Laid-Open No. 2004-145987, it is necessary to record a signal on an optical disk, to subsequently repeatedly drive and to set the spherical aberration correction mechanism for a number of times, and then, also to replay the optical disk correspondingly for a number of times, in order to adjust the spherical aberration. Similarly, it is necessary to repeatedly replay the optical disk to adjust the focus offset. The time necessary to drive the disk to make a full turn needs to be spent for each session of measuring the spherical aberration and the focus offset by way of such repeated recording and replaying operations.
An optical disk drive apparatus adapted to use a short wavelength (e.g., λ: 405 nm) and a high NA (e.g., NA: 0.85), as described above, is required to execute a process of adjusting and optimizing various parameters for recording/reproduction each time a disk is inserted. Then, the spherical aberration correction mechanism has to be driven for a number of times, and the optical disk needs to be replayed repeatedly in order to optimize the spherical aberration correction. Such an operation that needs to be conducted each time an optical disk is inserted is time-consuming and is inconvenient for the user.